Research is directed at understanding the cellular and genetic events that control normal T cell development. Transgenic and gene-targeting methods are used to analyze the function of known genes and various techniques (e.g.,RT-PCR) are being employed to identify novel genes that participate in thymocyte development. Current studies are focused on: (I) The role of the T cell antigen receptor (TCR) in thymocyte maturation. In mature T cells, the TCR transduces signals important for T cell activation and cell mediated immunity and in immature T cells the TCR directs thymocyte development and thymic selection. The TCR is composed of multiple signal transducing subunits (the CD3 chains and one or more members of the zeta-family of proteins; zeta, eta and FcepsilonRIgamma) that couple the TCR to intracellular signal transduction pathways. To address whether these signal transducing subunits perform distinct or analogous functions in development we have: a)examined the function of each of the individual subunits (zeta, CD3-epsilon, FcepsilonRIgamma,eta) by overexpression in transgenic mice; b)examined the role of zeta-chain in T cell ontogeny by generating zeta/eta-deficient and subsequently zeta/eta/FcepsilonRIgamma deficient mice, and c)examined the role of the 3 individual zeta-chain signaling motifs (Immunoreceptor Tyrosine based Activation Motifs; ITAMs) in thymocyte development by transgene reconstitution of zeta-deficient mice. These studies have revealed that expression of zeta chain (or a related protein) is required for TCR surface expression but that zeta chain signals are not specifically required for T cell development. Thus the CD3 subunits are sufficient to transduce all of the signals necessary for thymocyte maturation. Moreover, the zeta and CD3 subunits contribute to the overall signaling potential of the TCR and together determine the specificity of the T cell repertoire generated in the thymus. (II) The role of other signal transducing proteins in T cell development is being examined by the generation of transgenic mice. These include the CD3epsilon subunit of the TCR and CD5 a distinct surface receptor that also contains an ITAM. (III) Novel genes that have potential functions in thymocyte development or T cell activation are being identified. A new PTK, Txk has been cloned and biochemical and transgenic approaches are being employed to analyze its function. A similar approach is also being used to identify other genes involved in early thymus/thymocyte development.